As a stepping stone towards the energy efficient green living, investments for the exploitation of renewable energy resources are increasing worldwide, with particular attention to wind and solar power energy plants. But, the intermittence of these resources requires high efficiency energy storage systems. Today the most convenient form of power source are the electrochemical cells/batteries that provide portability for chemical energy storage and its conversion to electrical energy by electrochemical oxidation and reduction reactions which occur at the electrodes. Additional benefits appear in the form of zero emissions and high energy conversion efficiency.
In addition, there is currently an enormous research effort aimed at developing ultrathin, flexible and soft batteries to cater for the bendable modern gadgets. The goal is improved rate capability without a penalty in charge capacity, and sufficient electrochemical cycling characteristics. Flexible batteries are not only needed, e.g., for rolled-up displays, active radio-frequency identification tags, integrated circuit smart cards and implantable medical devices, but there is also the intention to place large flexible batteries in hollow spaces of the auto body of future hybrid and electric vehicles. Needless to stress that high power and high energy density are expected. Of course, the battery performance is closely related to the structural and electrochemical properties of the applied electrodes. Hence, the development of flexible electrodes with high energy and power density, good rate capability which can function safely for many years becomes important.
Li-ion batteries (LIB) are preferred over other systems because of long cycle life, broad temperature range of operation, low self discharge rate, high performance in terms of capacity and energy density and no memory effect. They are also referred to as rocking chair batteries as the lithium ions “rock” back and forth between the positive and negative electrodes as the cell is charged and discharged. Of the components, anode is one of the most critical parts in the proper functioning of the cells since it acts as a host for the Li ions. Not only, it should have a high Li insertion capacity, but also allow the insertion/de-insertion of Li with ease while retaining its structural stability for high cyclability and longer cell life.
Carbon is so far the most preferred material for LIB anode but its storage density, often called capacity, has a theoretical limit (in the case of graphite it is 372 mAhg−1). Different metals like Sn, Al, Si etc. have therefore been investigated that are capable of storing far more lithium per gram by alloying with the later. These are however intrinsically unstable during cycling due to pulverization that causes large volume expansions (>250%) thus affecting the structural integrity of the anode. Moreover, the anode materials that are prepared in the powder form are usually coated onto a copper current collector to make them conductive and mechanically robust. This limits the flexibility of the electrode and also adds to the dead weight of the cell.
It is postulated that nano-structured electrodes can display better cycle stability. The advantage of small particles can be explained by the low volume expansion and suppression of cracking and pulverization. Furthermore, small particle has fast electrode kinetics due to increased surface area. It should be noted that higher surface area will consume more Li for solid electrolyte interface (SEI) formation causing higher electrochemical irreversibility in the initial cycles. A more improved way to solve this problem is to use a second phase in order to accommodate the large volume change that occurs during charge/discharge cycles. Excellent conducting capabilities and small volume expansion for Li insertion make carbon an ideal matrix for lithium storage metals. It is expected that metallic nanoparticles uniformly dispersed and fixed on carbon can suppress their aggregation. Carbon coating can also restrain the electrolyte decomposition and provide integral and continuous conducting networks around the metal particles.
Among various kinds of carbon materials, the carbon nanotubes are attractive due to their unique structure, high electrical conductivity, high aspect ratio(>1000), remarkable thermal conductivity, good capacity and good mechanical properties. The advantages of this type of carbon nanotubes/metal composite are the increased capacity of the metal alloying materials while using the carbon nanotubes as a scaffold to prevent pulverization and crumbling in the anode. A compound made of both metal and carbon nanotubes has two mechanisms to store lithium with, intercalation and alloying. In addition to increased capacity and better cycling, carbon nanotubes can act as a conductive wire to transport electrons. Moreover, the high tensile strength, high flexibility and high aspect ratio (>1000) of carbon nanotubes make them uniquely suited for making free standing, flexible anode material for lithium ion cell.